1. Field of the Invention
This invention relates to a photometric control device for a camera in which a photometry is conducted by dividing a subject field into plural photometric regions, and particularly to a photometric control device using as a photoelectric conversion device an electric charge accumulation-type of photoelectric conversion device having plural picture elements which are matched with the number of divided photometric regions of the subject field.
2. Related Background Art
A photometric control device for a camera as disclosed in Japanese Laid-open Patent Application No. 2-280581 has been representatively known as a photometric control device for a camera in which a photometry is conducted on a subject field using an electric charge-accumulation type of photoelectric conversion device such as a CCD (Charge Coupled Device). This photometric control device uses a photoelectric conversion device serving as a photographing element to conduct a photometry. In this photometric control device, all of photoelectric conversion outputs which are obtained from plural picture elements constituting the photoelectric conversion device (corresponding to photometric signals for plural photometric regions of a subject field) are summed up, and an exposure time is controlled on the basis of the sum value.
On the other hand, Japanese laid-open Patent Application No. 62-259022 discloses a photometric control device using an integration type of photoelectric conversion device for focus-detecting to conduct a photometry. In this photometric control device, an integrating operation is conducted plural times using the photoelectric conversion device to obtain an average value of integration times of the respective photometric integrating operations, and the brightness value of a subject field is calculated on the basis of the average value.
A photometric brightness range which is generally required for a photometric control device for a camera is set to EV0 to EV20, that is, about EV20 in dynamic range. The dynamic range (a measurable brightness range in one photometry) of an electric charge accumulation-type of photoelectric conversion element such as a CCD available in the market is set to about 10 EV at maximum. However, in the electric charge accumulation-type of photoelectric conversion device, as the accumulation time is set to be longer, the measurable photometric brightness range is shifted to a lower brightness side. On the other hand, as the accumulation time is set to be shorter, the measurable photometric brightness range is shifted to a higher brightness side. Therefore, by controlling the accumulation time in accordance with the actual brightness of a subject field, the photometry can be accurately conducted on the subject field even if the photoelectric conversion device has a small dynamic range, and thus a proper exposure value can be calculated.
In the photometric control device as disclosed in Japanese Laid-open Patent Application No. 2-280581, the photometric signals for all the photometric regions of the subject field are summed up, and the accumulation time of the photoelectric conversion device is controlled in accordance with the sum value. Therefore, the accumulation time, that is, the photometric brightness range is necessarily set on the basis of average brightness in the subject field. This setting method of the accumulation time frequently induces a disadvantage that photometric signals at the low-brightness side or high-brightness side are not obtained even if photometry is conducted several times, and thus no proper exposure value can be obtained. Further, in the photometric control device as disclosed in Japanese Laid-open Patent Application No. 62-259022, no consideration is made to the dynamic range problem of the photoelectric conversion device as described above. Therefore, in this case, like the above case, there frequently occurs a case where the photometric brightness range is set to an undesired range, and thus no proper exposure value can be calculated.